Pocket cruisers

Discussion in 'Powerboats' started by Guillermo, May 13, 2006.

  1. tom28571
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    tom28571 Senior Member

    Guillermo,

    I will leave this to Tad to make an informed answer since it is a bit out of my experience. Off the cuff though, the bulbous bows that I have seen on small boats have not been very successful. Unable to show advantage in other than very low sea state and limited to narrow speed range, if at all. Just looking at the sketch, I might think that there would be some slamming in rough water with the nearly flat fore sections of the upper hull. In speculating about the box keel concept, fairing the fore sections into a form that will be reasonably comfortable in a large chop or waves seem to be critical in making the whole thing work well.

    I think that in order for the concept to be a satisfactory cruising boat, it should run comfortablly and economically at all speeds from zero up to the high teens.
     
  2. yacht371
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    yacht371 Yacht Designer

    Tom has a good point, the more volume there is in the keel, the flatter the upper sections get. It must be so, or the chines would be well out of the water and the boat would flop over. However, my experience with other hull type leads me to believe the the central part of the hull takes the brunt of the impact when falling off a wave, and the boat is already substantially decelerated by the time the upper sections hit. Neverthe less, this type suits bigger boats bets. It is also harder to build than some other hulls.

    I don't think I would recommend the powerkeel hull for the type of boat we are discussing here, since it as its best in speeds just above displacement range. The Flowmocean hull is probably the best fit. I have tank test data, which I have extrapolated to a 28' x 10' beam x 8000 lb. boat. The following assumes 65% prop efficiency.

    Knots HP GPH NMPG
    5.18 3 0.17 31.30
    6.29 6 0.31 20.01
    7.04 9 0.47 15.03
    8.15 15 0.77 10.61
    9.26 21 1.04 8.89
    10.00 24 1.21 8.25
    11.11 30 1.48 7.52
    12.22 36 1.79 6.82
    13.33 43 2.13 6.27
    14.44 51 2.55 5.67
    14.81 54 2.71 5.46

    The model test was limited by the speed capabilities of the towing tank. The odd intervals are the result of the scaling formulae. Extrapolating beyond these figures suggests 120 HP would give about 22 knots.

    Caveat: your mileage may vary. Tank testing and scaling have limitations. I have included factors for wind drag and hull roughness but these are very approximate. To be safe, I would add 25% or so to the required horsepower. Prop efficiency can vary a lot. Long thin blades like WHIOs are very effective, but rarely used because they take extra draft and you need a big reduction ratio or a slow turning engine.

    Grahame Shannon
     
  3. Guillermo
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    Guillermo Ingeniero Naval

  4. brian eiland
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    brian eiland Senior Member

    Larger Pocket Cruisers

    Just found this thread. It would be interesting to have "yacht371" the PowerKeel developer add his observations to this other thread.

    I had posted over on this new thread ,"Displacement Glider, PowerKeel, etc?" http://www.boatdesign.net/forums/showthread.php?p=95771
    Hello Mark,Stick to you guns lad, I like this concept of a marriage between the PowerKeel hull concept and Paine's Steadysailer design. It certainly offers the 'sailing alternative' to Dashew's FPB concept.

    And I think we will see more of these long-range passagemaker ideas evolve as the new world order of fuel prices become reality.

    And just to add more spice to the discussions I will reference this subject thread on a few other similar discussion threads
     
  5. yacht371
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    yacht371 Yacht Designer

    A lovely looking power trimaran that somehow manges to look traditionally yachty and ultra modern at the same time. Smaller than our target, but looks like it might scale well.

    http://www.kenau.nl/

    Grahame Shannon
     
  6. Zewe
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    Zewe Junior Member

    That is one sexy little trimaran!
     
  7. Vega
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    Vega Senior Member

    Very nice trimaran. The Design is very good. It is very dificult to design a tri or a cat to look like "in tradition" and at the same time looking modern too. This one looks just right;)
     
  8. tom28571
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    tom28571 Senior Member

    Once you decide that the disadvantages of a multihull powerboat are acceptable, such boats as this one can make a bit of sense. Trouble is, few have been willing to accept those negative aspects. If you just look at this handsome little launch, the problems related to making it into a viabler cruiser are apparent. Initial cost, slip cost, carrying capacity compared to monohulls of similar dimensions, trailerability, etc.

    A local guy built a larger power tri (about 26', I think) about 10 years ago. The main hull was beamier than this one, but still slim, with a V planing bottom and the amas were designed to ride on the wave generated by the main hull. Top design speed was about 50mph. I saw the boat but was not able to find out what happened to it.
     
  9. Guillermo
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    Guillermo Ingeniero Naval

    Very nice looking, indeed! (Although I wouldn't like to see this Keanu ferociously fighting the Spaniards....:D )
     
  10. yacht371
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    yacht371 Yacht Designer

    MC29 has arrived

    The Motorcat MC29 has arrived in Vancouver, and we have had it operating for a week. Performance figures and photos are at www.aviadesign.com/MC29

    The finish and construction is excellent, being the first one there are a few stowage enhancements needed. Performance is good, but I have to learn to steer as it is very different from my sailboat with its 6 foot deep rudder. That said, I haven't really had any trouble getting it to go where I want it to.

    Ride is excellent. We blasted through a big tide rip at 20 knots when bigger boats were slowing right down and smaller one becoming airborne. You could tell the water was rough mainly by the noise but G-forces were low and it was in no way a jarring ride.

    Grahame Shannon
     
  11. Guillermo
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    Guillermo Ingeniero Naval

    Nice results, Grahame. Less than 1 Lt/NM at 20 kn in loaded condition is very atractive.
    I find the wake to be somewhat 'dirty' as per the photos at your site. At what speed and load were they taken? Would it help to use chined hulls, instead of rounded, to avoid water going up the sides? Also, maybe rising the central hull at the stern out of the water would flatten the wake?

    Congratulations for your new boat. I wish you the best of lucks in the commercializing.
    Cheers.
     
  12. yacht371
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    yacht371 Yacht Designer

    Hi Guillermo,

    The wake IS a bit dirty looking at maximum speed (the photos were at 23+ knots) cleaner as you slow. The boat had 5 people on board, full tanks, and all my tools for installing equipment, so she was heavy. Wave propogation is small and the wake doesn't seem to disturb sailboats. We have developed spray rails for the bows which are not yet installed on this boat, we are waiting for them to arrive. These raise the bow a bit and clean up the spray patterns. Even without them, she is the driest boat of this size I have ever been aboard.

    Grahame Shannon
     
  13. Guillermo
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    Guillermo Ingeniero Naval

    Attached Files:

  14. yacht371
    Joined: Aug 2005
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    yacht371 Yacht Designer

    MC 29 video footage

    I have added video footage of the MC 29 in action at www.aviadesign.com/MC29

    The Tad Roberts design is a great looking boat, and should be economical to build. Too bad there don't seem to be any reasonably priced prodiction boats of this type.

    Grahame Shannon
     

  15. Guillermo
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    Guillermo Ingeniero Naval

    Nice video Grahame.
    Riding seems soft, indeed.
    How does she perform in tight turnings at high speed?
    And in beam seas?
    Cheers.
     
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